Abstract

We present results of a combined X-ray/optical analysis of the dynamics
of the massive cluster MACS J0358.8-2955 (z = 0.428) based on
observations with the Chandra X-ray Observatory, the Hubble Space
Telescope and the Keck-I telescope on Mauna Kea. MACS J0358.8-2955 is
found to be one of the most X-ray luminous clusters known at z > 0.3,
featuring LX, bol( < r500) = 4.24 ×
1045 erg s-1, kT = 9.55+ 0.58-
0.37 keV, M3Dgas( < r500) =
(9.18 ± 1.45) × 1013 M&sun; and
M3Dtot( < r500) = (1.12 ±
0.18) × 1015 M&sun;. The system's high
velocity dispersion of 1440+ 130- 110 km s-
1 (890 km s-1 when the correct relativistic equation is
used), however, is inflated by infall along the line of sight, as the
result of a complex merger of at least three subclusters. One collision
proceeds close to head-on, while the second features a significant
impact parameter. The temperature variations in the intracluster gas,
two tentative cold fronts, the radial velocities measured for cluster
galaxies and the small offsets between collisional and non-collisional
cluster components all suggest that both merger events are observed
close to core passage and along the axes that are greatly inclined with
respect to the plane of the sky. A strong-lensing analysis of the system
anchored upon three triple-image systems (two of which have
spectroscopic redshifts) yields independent constraints on the mass
distribution. For a gas fraction of 8.2 per cent, the resulting
strong-lensing mass profile is in good agreement with our X-ray
estimates, and the details of the mass distribution are fully consistent
with our interpretation of the 3D merger history of this complex system.

Underlining yet again the power of X-ray selection, our analysis also
resolves earlier confusion about the contribution of the partly
superimposed foreground cluster A 3192 (z = 0.168). Based on very faint
X-ray emission detected by our Chandra observation and 16 concordant
redshifts we identify A 3192 as two groups of galaxies, separated by 700
kpc in the plane of the sky. The X-ray luminosity and mass of the two
components of A 3192 combined are <0.5 per cent and <8 per cent of
that of MACS J0358.8-2955.